The present invention relates to a multitube collimator or the like to be placed between a radiation source and a radiation sensitive medium, for example for scintillation cameras, comprising a grid the walls of which are made of a radiation absorbing material and being moveable for setting in different angular positions.
During the last decade radionucelides (radioactive isotopes) have become more and more important as a facility in medicine when diagnosting and mapping the distribution of unhealthy processes in different organs. This depends partly on the fact that the number of available short duration radionucleides and radioactive pharmaseptica has increased and partly on the fact that progress has carried about effective instruments for the detection and registration of the gamma radiation from the organ or organs which has/have enriched the radionucleides which are delivered to the patient. One such instrument is the so called scintillation camera, often called the gamma camera, which was invented by Anger at the end of the fifties, and which is today spread in a great number all over the world.
A gamma camera is a relatively expensive instrument that in a basic version costs about 500,000 swedish kronor. Additionally, there is the cost of the surrounding equipment to which the so called collimators belong. A modern multitube collimator costs about 20,000 swedish kronor. In order to reach the best possible examination results in a given situation it is necessary to have a number of different colimators at hand.
The expensiveness and the possibilities to reach high quality examination results with the gamma camera generally lead to an effort to keep the frequency of use at a high level. Frequent change of collimators which thus is a nessecity for making use of the potential possibilities of the gamma camera is a circumstance that strongly contributes to keep the frequency of use at a low level or to decrease it.
The collimator is the image making element in a gamma camera. The geometric distance of resolution and the sensitivity are characteristic properties of a gamma camera. The geometric distance of resolution of the gamma camera is a measure of the accuracy that the collimator contributes to the number of details in the image that reproduces a certain distrubution of radionucleides. The sensitivity of the collimator is a measure of the transparency for gamma photons from a source of radionucleides.
The system distance of resolution of the gamma camera, which is a measure of the total resolving power of the gamma camera system, is a function of the internal geometric distance of resolution of the gamma camera and the geometric distance of resolution of the collimator. The internal distance of resolution of the gamma camera which is the minimum possible distance between two well collimated photon rays when they are separated in the image of the gamma camera is in modern gamma cameras of the same order as the geometrical distance of resolution of the collimator when the radionuclide source is near to the collimator.
For a parallel tube collimator the geometrical distance of resolution is inversely proportional to the length of the tube.
For a converging multi tube collimator, which causes a reduced wiew range, the geometrical distance of resolution is of the same order as for a parallel tube collimator. The influence of the internal distance of resolution is however reduced by increasing convergency. The sensitivity is larger than for a parallel tube collimator.
A collimator may also be divergent, which leads to a larger field of wiew, but to a reduced resolving power.
The system distance of resolution of the gamma camera is strongly influenced by the distance of resolution of the collimator.
The sensitivity of the collimator is strongly influenced by the geometrical design of the tubes, i.e. their diameter and length and the wall thickness and by the slope of the tubes.